When terminating an electrical conductor which is intended to carry relatively high current at relatively high frequency, care must be taken to keep current density at the termination within reasonable limits so that high resistance paths are not inadvertently created. Typically, such high current high frequency applications include high energy pulses for laser devices where the pulse rise time is less than one nanosecond per volt, and induction heating systems and other electromagnetic systems requiring high frequency energy of 10 kilohertz or more.
Such systems may, for example, require continuous current flow of as much as 5,000 amperes with peak demands of 20,000 amperes for short periods of time. Due to short rise time pulses or high frequency alternating current, current flow through the conductor is limited to a portion of the conductor close to its surface. This phenomenon is known in the industry as "skin effect." Additionally, self and mutual inductance of the conductors acts as a choke, further limiting current flow at certain frequencies.
"Skin effect" is the terminology used to describe the tendency for alternating currents to concentrate and flow in the outer region of a conductor. This outer region is defined by the "skin depth" such that, for a circular cross-section, this depth is measured inward from the conductor's surface. Most of the total current flows within this region. Therefore, one finds that the AC resistance of a wire is greater than the DC resistance of that same wire due to the reduced effective cross-sectional area through which the current must pass. The precise value of the skin depth for a circular cross-section is defined as: ##EQU1## where: f=frequency
.mu.=permeability PA1 .sigma.=conductivity
Attempts to overcome these problems by utilizing multiple conductors in parallel have met with some success. Multiple insulated conductor strands in close, parallel proximity exhibit about a 50% reduction of inductance over a single strand conductor. Such multi-strand conductors, now commercially available, are typically composed of thousands of very small diameter wires, each of which is insulated by a thin coating of varnish such as polyimide, or some other insulating material such as epoxy.
Such multi-strand cables being used for the transmission of high frequency and high current energy, are often subjected to high ambient temperatures due to the nature of the devices utilizing the transmitted energy. Induction heating systems, for example, frequently produce an ambient temperature near the power cable of about 300.degree. C. This relatively higher temperature causes a decrease in conductivity which, in turn, causes an increase in the skin depth. For example, using a copper conductor and current at 10 kHz, a change from room temperature to 300.degree. C. will result in about a 45% increase in skin depth.
Connectors for removably connecting these cables to their respective equipment must, therefore, be able to transmit the high frequency, high current energy without imposing high resistance paths.
One current practice of terminating a multi-strand conductor is to solder the conductor to a terminal. This results in a rather long section of the cable being saturated by the solder which reduces the length of uninsulated wire strands, making them, in effect, into a single conductor with higher inductance, yielding a high resistance path for the AC current. This occurs because a chemical solution is used to strip the insulation from the individual strands of the conductor, and due to capillary action, the solution is carried far into the bundle of twisted wire strands. Similarly, the solder is drawn into the bundle of strands to the extent of the stripped wire. Another current practice is to fuse the ends of the bundle of strands to an adjacent terminal. This solves the problem of a solder saturated cable, but still results in a high resistance path at the fused junction of the wire strand ends and the terminal because of the aforementioned "skin effect" phenomenon of the limited area of contact. Examples of such terminations are disclosed in Swengel Sr. et al. U.S. Pat. No. 3,656,092 which issued Apr. 11, 1972, and Lin et al. U.S. Pat. No. 4,042,775 which issued Aug. 16, 1977. Such terminations work well for D.C. or low frequency applications, however, at high frequencies, due to skin effect, the cross-sectional area available for current flow is reduced. This is known in the industry as constriction resistance.
What is needed is a termination for connecting multi-strand cables to their respective equipment in high frequency, high current, and high temperature applications without adversely affecting current flow by constriction.